The invention relates to the field of optics, and in particular to the formation of mode-size fiber-waveguide couplers.
One of the characteristics of integrated optical devices is that they are based on single-mode high index-contrast waveguiding, which enables the dense integration of these devices on an optical chip. A major disadvantage of this approach is the difficulty of coupling light to and from an optical fiber. By approximating the fiber mode by a gaussian distribution, the mode field diameter (MFD) of the fiber is defined as the diameter of the gaussian power distribution, and is approximately 15% larger than the core diameter.
For a single-mode fiber (SMF) with a flat-end, the typical MFD is between 8 and 10 xcexcm, and the mode cross-section is ideally circular. A lensed fiber has an output beam diameter of approximately 50% of the typical MFD and approximately a 20 xcexcm focal length. A high index-contrast waveguide has a fundamental mode in the submicron range, depending on the aspect ratio of the rectangular waveguide core, the mode cross-section is strongly elliptical. This large mode-mismatch leads to a very inefficient fiber-waveguide coupling where most of the power is lost to radiation. The coupling loss between fibers of different MFD, assuming that they are perfectly aligned, is defined as                               loss          ⁢                      xe2x80x83                    ⁢                      (            dB            )                          =                              -            10                    ⁢          log          ⁢                      xe2x80x83                    ⁢                      {                          4                                                (                                                                                    MFD                        1                                                                    MFD                        2                                                              +                                                                  MFD                        2                                                                    MFD                        1                                                                              )                                2                                      }                                              Eq        .                  xe2x80x83                ⁢        1            
If the ellipticity of a waveguide is ignored and Eq. 1 is applied to get an estimate, the loss associated with the coupling between a fiber and a waveguide with 5-to-1 MFD ratio will be approximately 8.3 dB, which is less than 15% coupling efficiency. This is illustrated with a 2D numerical example hereinbelow. The numerical method used is the Finite Difference Time Domain (FDTD).
FIG. 1A is an electric field diagram and FIG. 1B is a graph of the transmission and reflection response illustrating coupling between a low index-contrast wide waveguide to a high index-contrast waveguide. FIG. 1A shows the low index-contrast wide waveguide having an index of approximately 1.05, a width of 4 xcexcm, and a MFD of approximately of 4.8 xcexcm, which is coupled to a high index-contrast narrow waveguide having an index of 3 and a width of 0.25 xcexcm. The whole system is surrounded by air (n=1). FIG. 1B demonstrates that most of the power is lost to radiation and only 17% of the power is coupled into the waveguide mode.
Most approaches to solving this problem can be broadly classified into to two types depending on whether the coupling schemes reside on the fiber side or the chip side. In the first type of coupling, a fiber tip is modified by tapering and/or lensing to bring the MFD of the fiber mode closer to that of the integrated waveguide. In the second type of coupling, the core of the integrated waveguide, is adiabatically tapered so that the mode fields spreads out into the cladding to match the fiber mode size. Both types of coupling structures have lengths that are a few hundred microns.
Mode conversion schemes that work entirely on the fiber side can lead to critical alignment tolerances as the fiber mode size gets very small. Moreover, there is still a mis-match due to the different mode-shapes, because the fiber being circular and the waveguide being highly elliptical. For these reasons, it is preferable to concentrate most or all the mode matching efforts on the chip-side. Better alignment tolerances are obtained and the added advantage that one kind of fiber can be used to couple light into different photonic integrated circuits (PICs).
According to one aspect of the invention, there is provided a planar lens coupling system. The planar lens coupling system includes an input fiber providing an input beam of a defined mode size. A first interface layer of a defined index, the first layer includes the radial length of the planar lens. A high index-contrast waveguide is coupled to the first interface layer. The high index-contrast waveguide has the same index as the first interface layer and a mode size that is smaller than the input beam. A second interface layer of a defined index is coupled to the first interface structure. The second interface layer is formed on the propagation axis as impedance matching at the location of maximum intensity. The first and second interface layers lower the mode size of the input beam laterally, thus providing improved coupling between the input fiber and the high index-contrast waveguide.
According to another aspect of the invention, there is provided a 3D coupling system. The 3D coupling system includes a layered structure that receives an input of a defined mode size. The layered structure includes a plurality of layers with varying indexes, and outputs a vertically mode converted beam associated with the input beam. A planar lens structure receives the vertically mode converted beam, and performs lateral mode conversion on the vertically mode converted beam. The planar lens structure outputs a laterally and vertically mode converted beam. A high index-contrast waveguide structure receives the laterally and vertically mode converted beam, and provides the laterally and vertically mode converted beam to a chip device. The high index-contrast waveguide has a mode size smaller than the input beam.
In still another aspect of the invention, there is provided a 3D coupling system that simultaneously performs vertical and lateral mode size conversion. In the vertical dimension of the 3D coupling system includes of a layered structure with quadratically varying refractive index that vertically converts the input fiber mode-size to match the vertical mode-size of the output high index contrast waveguide. In the lateral dimension of the 3D coupling system includes as a planar lens structure. The planar lens structure includes a curved interface. The curved interface has a radius that is chosen to laterally convert the input fiber beam to match the lateral mode-size of the output high index contrast waveguide.